onclickIn fact, you can directly bind functions with parameters, as follows:

const input = document.getElementById('input');
input.onclick = function(params){
  console.log(params);
};

However, please note that your requirement is to execute the function fn(p), and the result of the execution is to return a value, not a function.
fn(p)，执行的结果是返回一个值，而不是一个函数。
onclickAs a callback event when clicked, if bound, it must be bound to a function, not a value. As follows:

input.onclick = function(){
  fn(p);
};

What this means here is to bind an onclick事件，这个事件是一个函数，点击时，回调就被执行了，意味着函数也被执行了，函数执行时，其中的语句fn(p) event to the input. This event is a function. When clicked, the callback is executed, which means that the function is also executed. When the function is executed, the statement fn(p ) is executed.

onclick = function(e) {console.log(e)}

Try to bind this click function and see the result